The present invention relates to an apparatus for controlling the retard angle of ignition timing of an internal combustion engine, or more in particular to an apparatus for minimizing an audible knock in an internal combustion engine by retarding ignition timing, when necessary, from the normally set ignition timing in response to the signal from an engine-mounted vibration sensor.
The knock generated in an engine, as well known, reduces the torque and the operating efficiency of the engine or damages the engine by overheat. This knock is greatly affected by the ignition timing. The output of the engine at a given speed increases with the advance of the ignition timing, that is, with the increase in advance angle. With the increase in advance angle, however, a knock occurs undesirably at a certain time point. If the advance angle is reduced excessively to avoid the knock, on the other hand, the output is reduced. The ignition timing, therefore, is required to be regulated to occur immediately before the time point at which a knock will occur. In order to meet this requirement, the knock conditions are detected so that the ignition timing is controlled in accordance with the generation of knocks.
In this knock control, the occurrence of a knock is generally detected by an engine-mounted vibration sensor. This vibration sensor detects engine vibrations and produces an output corresponding to the vibrations in the form of an electrical signal. An engine vibration is a combination of not only a knock but also the inherent vibration of the engine, vibrations due to the explosion of the air-mixed fuel at the time of ignition and other vibrations. The vibration due to the explosion at the time of ignition is very great and eliminated from the output of the vibration sensor by nullifying the output per se or the like. The output of the vibration sensor is thus divided roughly into a knock signal component based on a knock or a similar vibration having a large amplitude especially at or in the vicinity of a characteristic frequency and a background signal component having a small amplitude distributed over a wide frequency range and based on the inherent vibrations of the engine or the like.
In conventional methods of knock detection, the output signal of the vibration sensor is applied to a band-pass filter which passes only the frequency range including the characteristic frequency at the center thereof, and the output signal of the band-pass filter is compared with a reference voltage. The output signal of the band-pass filter is a combination of the knock signal component and the background signal component in the presence of a knock, while it contains only the background signal component in the absence of a knock. The reference voltage applied to the comparator is set at a level higher than the peak of the background signal component, which level is exceeded by the peak of a signal composed of the knock signal component and the background signal component in the presence of a knock, thereby making it possible to detect the existence of a knock.
A prior art and an improvement thereof for detecting knock by the use of a band-pass filter and a comparator for processing the output of the vibration sensor as mentioned above is disclosed in U.S. Pat. No. 4,111,035 issued to West et al. on Sept. 5, 1978 and assigned to General Motors Corporation. In West et al., in addition to the band-pass filter and the comparator in the prior art there is further provided, as means for correcting mistuned states with respect to the characteristic frequency of the vibration sensor, a low-pass filter through which the output of the comparator is fed back to the input of the same comparator. In the conventional methods including West et al., however, the output of the band-pass filter is directly applied to the comparator, thereby posing a practical problem of low knock detecting accuracy. In view of the fact that an increased gain of the band-pass filter causes a distortion of the output thereof, the gain thereof is unavoidably reduced. When the output of the band-pass filter is small, the direct application thereof to the comparator for comparison with a reference value may cause a malfunction in which a knock fails to be detected.